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Different Types of QFNs-Advantages-Disadvantages-pcb

Different Types of QFNs-Advantages-

Disadvantages-pcb

Different Types of QFNs

Two types of QFN packages are common: air-cavity QFNs, with an air cavity designed into the package, and plastic-moulded QFNs with air in the package minimized.

Less-expensive plastic-moulded QFNs usually limited to applications up to ~2–3 GHz. It is usually composed of just 2 parts, a plastic compound and copper lead frame, and does not come with a lid.

In contrast, the air-cavity QFN is usually made up of three parts; a copper leadframe, plastic-moulded body (open, and not sealed), and either a ceramic or plastic lid. It is usually more expensive due to its construction, and can be used for microwave applications up to 20–25 GHz.

QFN packages can have a single row of contacts or a double row of contacts.

Advantages

This package offers a variety of benefits including reduced lead inductance, a small sized "near chip scale" footprint, thin profile and low weight. It also uses perimeter I/O pads to ease PCB trace routing, and the exposed copper die-pad technology offers good thermal and electrical performance. These features make the QFN an ideal choice for many new applications where size, weight, and thermal and electrical performance are important.

Disadvantages

The small size of the exposed contacts, and the large area of exposed thermal pad makes it easy for small parts, such as 3x3 mm DFN packages, to float on the pool of molten solder under the thermal pad during assembly. This causes the parts to make no contact to the printed circuit board pads in some instances. Due to the excellent thermal characteristics of this mounting package, it is very hard to rework the device, as hot air reflow typically does not offer enough heat to the thermal pad without damage to surrounding board material or parts. Oxidation of the exposed chip contact pads after being exposed to a reflow oven during initial assembly makes solder wetting to them during rework quite difficult. Additionally there is no clearance for a soldering pencil to reflow pads under the chip if touch up is desired. Sometimes contact can be made up the sides of the DFN package contact pads, but this does not work well in practice.


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